1. Field of the Invention
This invention relates to a device for optical switching with extended travel range.
2. Description of Related Art
The shift of the Internet from the government and university realm into the public, commercial sphere has created unprecedented levels of growth in the demand for network services. Much of the existing network infrastructure is unable to meet these demands. Typically, these systems multiplex 45 Megabit per second (Mb/s) electrical signals to higher bit rates and transmit them on a single optical fiber. The recent development of dense wavelength division multiplexing (DWDM) systems has allowed these systems to expand by transmitting more than sixteen wavelength channels of data on a single fiber, at 2.5 gigabit per second Gb/s per channel. Currently, the latest commercially available DWDM systems allow up to 160 channels at 10 Gb/s per channel but systems under development using standard single mode optical fibers have demonstrated 3.2 Terabit per second Tb/s with 80 channels at 40 Gb/s per channel. However, this extraordinary expansion of network capacity must be matched by a commensurate expansion of network management infrastructure.
Existing optical networks require frequent conversion of the optical signals to the electrical domain and back again at repeaters and add/drop multiplexers with associated high cost and loss of flexibility. This has led to the vision of an all-optical xe2x80x98photonicxe2x80x99 network architecture, where signal amplification and switching functions are all performed at the optical layer. When considered together with the exponential growth of network bandwidth, the need to develop high performance, low cost active optical elements is desirable. Such elements include tunable lasers and filters, optical attenuators and switches. Currently all of these elements are built with silicon micro-machining (MEMS) techniques. MEMS based free space optical elements have the advantage of lower signal degradation than waveguide technology, reducing the requirements for optical amplification, and therefore lowering the system cost.
The micro-machined versions of the optical elements listed above all have at their core the common element of a movable reflective membrane. In order to achieve low insertion loss, that is, low signal attenuation, with typical beam divergences, and system geometries, the reflective surfaces must be rather large, on the order of 700xc3x97700 Mm2. However, large flat mirrors cannot be easily fabricated using conventional silicon deposition and micro-machining technology because the mirror surface, which is composed of gold (Au) on polycrystalline silicon (Si) for wavelength independent reflectivity near 1.5 Mm, constitutes a bimorph structure which is generally highly non-planar. In particular, the mirror will deform with variations in temperature, resulting in poor performance. While it is possible to work around these problems by careful design and stress engineering, a simpler solution is to make the device insensitive to stress by using an extremely thick silicon layer. This also allows the development of highly reflective or wavelength selective devices based on dielectric stacks, where the stresses would be more difficult to compensate.
Micromachined, electrostatically actuated mirrors can be used as variable attenuators, modulators and switches in optical networking systems. Fundamental issues such as the trade-off between extended travel range and low pull-in voltage need to be addressed in order to provide increased design freedom, for customized applications. In theory, the maximum travel range before snap down for full-plate electrostatic actuators is 44% of the full-scale deflection. The prior art has employed leveraged bending and strain stiffening in an effort to extend this range. Representative examples of the prior are contained in U.S. Pat. No. 5,589,974 issued to Goossen and U.S. Pat. No. 5,923,798 issued to Aksyuk
The disclosed device relies on a combination of series dielectric layers and intrinsically trapped electrostatic charges to provide a completely electronic means for extension of the range of travel for the micro mirror.
The invention relates to a micro-machined, electrostatically-acutated optical attenuator/switch fabricated by fusion bonding of single crystal, ultra-thin silicon wafers. The travel range of the reflective element is extended by a dielectric layer on the under side of the reflective element and a corresponding dielectric layer on the substrate below and facing the reflective element. The dielectric element is fabricated so as to have a permanent electrical charge. The two facing dielectric layers have identical electrical charge so that when the reflective element flexes toward the substrate, electrostatic forces act to repel the reflective element. In addition, oxide charge of each electrode acts in parallel with the electrostatic attraction, in the case of similarly charged electrodes decreasing the effective actuation voltage and increasing the travel range before snap-down. The oxide capacitance acts as a voltage divider, lowering the effective actuation voltage across the gap of the mirror structure. The insulating properties of the dielectric layers, absent a charge, also provide some degree of travel extension and inhibit electrode shorting. The net result is to extend the angle over which the reflective element may travel before it snaps down and sticks to the substrate layer, known as stiction. The device contains a large area, electrostatically actuated micro-mirror for use as a variable attenuator or switch in optical networking systems. An embodiment shows the device with a reflective surface ranging from 400 to 700 micrometers square and from 2 to 200 microns thick with two torsional springs mounted on adjacent comers. The torsional springs may contain from 1 to 3 elements depending on the application and the desired spring constant. In alternate embodiments the reflective surface may be in the shape of a circle. The reflective element is a front surface mirror coated with a thin reflective coating to bounce the incoming light beam back to a receiver. A bias voltage is applied across the reflective coating on the reflector and the silicon substrate. The reflective surface deflects toward the substrate when the bias voltage is applied. The angle at which the reflective surface deflects the incoming beam is determined by the bias voltage value. As deflection increases and the micro-mirror gets closer to the substrate, the eletrostatic repulsion increases. Thus as deflection increases, a given incremental deflection requires a larger bias voltage to achieve the same deflection. A deflection angle of up to 1 degree has been achieved with the device. Eventually, the electronic attraction due to the bias voltage overcomes the electrostatic repulsion of the intrinsic charge and the mirror sticks to the substrate. This phenomenon is referred to as stiction.
The thickness of the reflective element is such that it remains flat over the desired range of deflection angles.
The devices are fabricated by fusion bonding ultra-thin, defined as less than 200 microns, single crystal silicon wafers to a micro-machined silicon substrate. This forms robust, non-deforming, reflective surfaces which are simpler to fabricate than similar devices fabricated by conventional chemical vapor deposition of polycrystalline silicon, which require careful engineering to avoid stress-induced deformation. Deep Reactive Ion Etching DRIE is used to form the torsional spring structures and the outline of the reflective surface. Dry etching employing Reactive Ion Etching (RIE) is then used to etch through the silicon oxide layer on the underside of the reflective element to free the reflective structure from the surrounding material, while leaving the oxide layer intact on the underside of the reflective element and the substrate. Finally, a Chromium layer is deposited on the silicon reflective element and a layer of gold is used as the reflecting surface.